Image projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
The following description is presented with reference to a color image projection system implemented with a color wheel but is applicable to other field sequential image projection systems. Color image projection systems operate on the principle that color images are produced from the three primary light colors: red (“R”), green (“G”), and blue (“B”). With reference to FIG. 1, a prior art image projection system 100 includes a primary light source 102 positioned at the focus of a light reflector 104 and emitting light having multiple wavelength bands that propagate in a direction away from light source 102 along a beam propagation path 106 through an optical integrating device 108, of either a solid or hollow type, to create at its exit end a uniform illumination pattern. The uniform illumination pattern is incident on a rotating color wheel 110. An exemplary color wheel 110 includes three regions, each tinted in a different one of primary colors R, G, and B. Light exiting color wheel 110 is imaged by a lens element system 112, reflected off a light reflecting (or transmitting) imaging device 114, and transmitted through a projection lens 116 to form an image. Popular commercially available image projection systems of a type described above include the LP300 series manufactured by InFocus Corporation, of Wilsonville, Oreg., the assignee of this application.
There has been significant effort devoted to developing image projection systems that produce bright, high-quality color images. However, the optical performance of conventional image projection systems is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projection systems in a well-lighted room.
Loss of image brightness can, in part, be attributed to the fact that typical image projection systems can utilize only portions of the light beam that are of a specified polarization state or of the color that corresponds to the region of the color wheel aligned with the primary light path at the time of incidence of the light beam on the color wheel. Portions of the light beam that do not correspond to the region of the color wheel aligned with the primary light path at the time of incidence are discarded from the image projection system. As a result, about 60% of the polychromatic light emitted by the primary light source is wasted because it does not pass through the color wheel. This 60% loss of light translates to a significant decrease in image brightness.
One attempt to increase image brightness involved recirculating polychromatic light in the optical integrating device, which was typically a light tunnel 108a, while implementing a spiral color wheel having three color regions simultaneously aligned with the primary light path. With reference to FIG. 2, a spiral type color wheel 110a includes R, G, and B dichroic coatings arranged in a “spiral of Archimedes” pattern defined by the equation R=aθ. Spiral color wheel 110a is located adjacent to an exit end 132 of light tunnel 108a, and the three color regions move at a nearly constant speed in the radial direction. The spiral color wheel 110a may also include a white region that can be used to increase luminous efficiency in non-saturated images. With reference to FIG. 3, spiral color wheel 110a is positioned such that light exiting the exit end of light tunnel 108a is simultaneously incident upon all of the color-selective regions of spiral color wheel 110a. Further, light tunnel 108a includes an entrance end 130 having an entrance aperture through which light emitted by light source 102 propagates. An inner wall 118 of entrance end 130 includes a highly reflective mirror that reflects light that is incident on and reflected by spiral color wheel 110a. Thus light is recirculated in light tunnel 108a. While highly reflective inner wall 118 facilitates light recirculation, the image projection system suffers a 60% reduction of input etendue due to the requirement that approximately 60% of the area of inner wall 118 of entrance end 130 is covered such that approximately 60% of the light emitted by light source 102 does not enter light tunnel 108a. In image projection systems implemented with all but the shortest arc lamps, the efficiency loss due to the etendue reduction is greater than the efficiency increase due to light recirculation within light tunnel 108a. High brightness projectors require high-power arc lamps which have arc gaps too large for this prior art method of light recirculation to be of significant value. Further, this attempt did not work with more distributed light sources such as electrodeless microwave discharge lamps.
What is needed, therefore, is an image projection system that exhibits increased optical efficiency and that is implemented with an improved technique for achieving increased image brightness without a significant reduction in etendue.